This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2001-374078 filed on Dec. 7, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel injection pump for an internal combustion engine (hereinafter called xe2x80x9cenginexe2x80x9d) in which mutual sliding contact portions of a cam ring and a plunger are well lubricated.
2. Description of the Prior Art
A conventional fuel injection pump for a diesel engine has a cam for driving a plunger as a movable member. In this pump, fuel is sucked and pressurized in a pressure chamber by reciprocating movement of the plunger axially slidable in a cylinder. A rotating movement of a drive shaft to be driven by an engine is converted to the reciprocating movement of the plunger inside the cylinder via the cam connected with the drive shaft and a cam ring disposed between the cam and the plunger.
To improve engine output and fuel consumption and to reduce emission such as NOx and black smoke to be exhausted from the engine, higher fuel injection pressure has been recently demanded.
To secure the higher fuel injection pressure, it is necessary to increase pressure of fuel to be pressurized by and discharged from the fuel injection pump so that higher load is applied to the fuel injection pump. In particular, larger force acting on mutual sliding contact portions of the cam ring and the plunger is likely to cause frictional seizure between the sliding contact portions. Therefore, a part of fuel is bypassed and supplied to the sliding contact portions of the cam ring and the plunger for lubricating the sliding contact portions with an oil film to be formed by the fuel thus supplied.
However, when the plunger is in a compression stroke during which fuel in the pressure chamber is pressurized, the plunger receives a large reaction force acting toward the cam ring from the fuel to be pressurized in the pressure chamber so that the plunger comes in close contact with the cam ring. Further, when the plunger is in an intake stroke during which fuel is sucked into the pressure chamber, the plunger is also urged toward the cam ring by biasing force of a spring so that the plunger comes in close contact with the cam ring, similarly as in the compression stroke. Accordingly, fuel for lubrication does not sufficiently enter between the sliding contact portions of the plunger and the cam ring, which tends to cause the frictional seizure between the sliding contact portions since the oil film therebetween for lubrication is scarcely formed.
An object of the present invention is to provide a fuel injection pump in which oil film is easily formed between sliding contact portions of a plunger and a cam ring so that frictional seizure therebetween hardly occurs.
To achieve the above object, in a fuel injection pump having a drive shaft, an eccentric cam integrated with the drive shaft, a cam ring arranged around outer circumference of the cam shaft, a housing provided with a cylindrical bore and a movable member axially movable in the cylindrical bore, the cam ring is provided on outer circumference thereof with a sliding surface. The movable member is biased toward the drive shaft so that an axial end thereof is in contact with the sliding surface. Another axial end of the movable member and the cylindrical bore form a pressure chamber. The movable member not only moves axially toward the drive shaft to suck fuel into the pressure chamber and but also moves axially in a direction remote from the drive shaft to pressurize the fuel in the pressure chamber, while the axial end of the movable member slidably and reciprocatingly moves relatively to the sliding surface, according to movement of the ring cam driven by the drive shaft via the cam.
With the fuel injection pump mentioned above, only a part of the axial end of the movable member comes in contact with the sliding surface on one side of an axis of the drive shaft so that a gap is formed between the axial end of the movable member and the sliding surface on the other side of the axis of the drive shaft, in an intake stroke when the fuel is sucked into the pressure chamber, and a substantially entire part of the axial end of the movable member comes in contact with the sliding surface on both sides of the axis of the drive shaft, in a compression stroke when the fuel in the pressure chamber is pressurized. Since high load is not applied to the movable member in the intake stroke, the gap between the axial end of the movable member and the sliding surface of the cam ring does not cause any problem.
To the contrary, in the compression stroke when the high load is applied to the movable member via the cam and the cam ring from the drive shaft for pressurizing the fuel in the pressure chamber, the mutual sliding contact portions of the movable member and the cam ring can be well lubricated with the fuel entered the gap in the intake stroke.
It is preferable that height of the gap is relatively low but larger than that of each surface roughness of the axial end of the movable member and the sliding surface to an extent that an oil film by fuel is sufficiently formed between the axial end of the movable member and the sliding surface for preventing frictional seizure of mutual sliding contact portions of the movable member and the cam ring.
Further, it is preferable that another cylindrical bore, another sliding surface, another movable member and another pressure chamber, whose constructions are similar as the cylindrical bore, the sliding surface, the movable member and the pressure chamber and each of the another cylindrical bore, the another sliding surface, the another movable member and the another movable member is arranged on an opposite side of each of the cylindrical bore, the sliding surface, the movable member and the pressure chamber with respect to the drive shaft.
In this case, when the part of the axial end of the movable member comes in contact with the sliding surface for sucking the fuel into the pressure chamber in the intake stroke, the substantially entire part of the axial end of the another movable member comes in contact with the another sliding surface for pressurizing the fuel in the another pressure chamber in the compression stroke.
In more details, the sliding surface and the another sliding surface are formed in non-parallel. As an alternative, the sliding surface and the another sliding surface may be provided respectively with a projection and another projection onto which the movable member and the another movable member run when the fuel is sucked into the pressure chamber and the another pressure chamber, respectively. Each of these constructions is effective to form the gaps between the axial end of the movable member and the sliding surface of the cam ring and between the axial end of the another movable member and the another sliding surface of the cam ring in the intake stroke. Accordingly, the oil film formed by the fuel serves to prevent the frictional seizure of the sliding contact portions between the movable member and the cam ring and between the another movable member and the cam ring.